Thermal ink jet printers use printheads containing heating elements on a semiconductor substrate for heating ink so that the ink is imparted with sufficient energy to cause the ink to be ejected through a nozzle hole in a nozzle plate attached adjacent to the substrate. The nozzle plate typically consists of a plurality of spaced nozzle holes which cooperate with individual heater elements on the substrate to eject ink from the printhead toward the print media. The number, spacing and size of the nozzle holes influences the print quality. Increasing the number of nozzle holes on a printhead typically increases the print speed without necessarily sacrificing print quality provided the ink is ejected at precisely the correct spot onto the media. However, there is a practical limit to nozzle hole or orifice size and to the size of the semiconductor substrate which can be produced economically in high yield. Thus, there is a practical limit to the number of corresponding nozzle holes which can be provided in a nozzle plate for a printhead.
For color printing applications, the three primary colors of cyan, magenta and yellow are used to create a pallet of colors. Typically, each color is associated with a nozzle plate and semiconductor substrate specifically designed or tuned to give optimal performance with the associated color. Such nozzle plates are typically attached to separate printheads so that the number of nozzle holes per color is maximized for high quality, high speed printing. However, it is extremely difficult to maintain an alignment tolerance of a few microns between the printheads when using separate printheads.
Using a single substrate containing separate heating elements for each color reduces the alignment problem associated with using separate printheads but reduces the number of nozzle holes and thus the print speed because of the practical limit to substrate size. In order to obtain suitable substrate production yields, the substrates or chips cannot be large enough to contain the same number of energy imparting devices as would be located on individual substrates attached to separate printhheads.
While locating multiple individual substrates of a conventional size on the same printhead allows relatively faster printing rates, such a design contributes to significantly increasing the printhead temperature because of the greater number of energy imparting devices located on the printhead and the desire to eject the ink from the printhead at a faster rate. The increased printhead temperature causes changes in the printhead dimensions making it difficult to maintain the spacing between multiple chips on the printhead thus adversely affecting print quality.
Various materials and methods have been proposed for removing heat from the printhead substrates. Conventionally, materials which exhibit a low thermal expansion coefficient have been used to provide suitable heat removal without sacrificing print quality. Materials having low thermal expansion coefficients do not typically expand or contract a sufficient amount to affect printer operation and thus print quality. These materials enable printhead designs that are tolerant of temperature variations since expansion and/or contraction of the components and electrical connections therebetween is minimized. However, such materials are typically made from exotic composite materials such as metal-ceramic mixtures, carbon fiber, or graphite composites which are costly to make and use in such applications.
Accordingly, an object of the invention is to provide a cost effective material for heat removal from printhead substrates without sacrificing print quality.
Another object of the invention is to provide a method for improving print quality in a multi-color printhead.
A further object is to provide a multi-color printhead for thermal ink jet printer which provides improved print quality at a relatively lower cost than conventional printheads.
A still further object of the invention is to provide a printhead and associated method which enables compensation for dimensional changes of the printhead so that print quality is not adversely affected by such dimensional changes.